How to join two UV maps pieces into one
Point 1, You can join seams.

Simply select the two vertices you would like to combine.

Then press W, then "Weld". This will "join" those two vertices.





Point 2, You NEED seams!

Unwrapping requires cutting the model up into pieces (for it to work properly). Imagine it like you have a paper (3D) figure and you have to let it lay out flat. You must cut it enough times to do this. If you mark a seam, that will be a split. If you have marked all the seams you want and press U, then "unwrap", it should unwrap with only your given seams, but after you mark a new seam, you must re-unwrap for it to take effect.



Point 3, Smart UV Project will automatically add new seams.

Smart UV Project is very automatic. It adds new seams where it deems best. This can be a problem if you are working with detailed unwraps. It is often easier to manually unwrap than welding seems left by Smart UV Project.



Point 4, You can do this for a whole seam.

Simply align the seams as best you can, then select them. (Tip, use proportional editing on "connected mode" to drag an edge closer to another while smoothly transforming the geometry in between.)

Then press W, then "Align Auto" to line up the two seams.

Now press W again (with both seams still selected) and click "Remove Double UVs". This will remove the UVs that are next to each other. If you succeeded, this should merge your seams.

Note: If the vertices were too far apart after aligning, this will not work. You can do any necessary clean up using weld.



Here is a more complex join.